1. Technical Field
The present invention pertains to a method and apparatus for attaining improved sound quality and extended range in wireless digital communication systems.
2. Discussion of Prior Art
Helmkamp et al (U.S. Pat. No. 5,265,150) disclose a wireless private branch exchange (PBX) system capable of automatic optimal configuration. Specifically, the wireless system includes a central transceiver, a plurality of fixed stations, and a plurality of portable stations capable of being carried throughout the site or area where the system is installed. Fixed stations are capable of being used as repeaters at which information from the central transceiver is received and retransmitted in a prescribed protocol to thereby extend the effective range of the central transceiver. Communications between the central transceiver and fixed stations are in a time division multiplexing (TDM) format allowing for implementation using only a single transmitter/receiver pair in each of the fixed and portable stations. The central transceiver and fixed stations are capable of selecting an optimal channel and utilize a time division duplexer to coordinate transmission and reception of signals.
The extended range of the central transceiver is divided into range levels wherein repeaters (fixed stations) situated at one level may communicate with only a single repeater of adjacent levels. Further, repeaters store and repeat certain frames destined for additional repeaters and range levels.
Bohm (U.S. Pat. No. 5,133,001) discloses a radio telephone system in the form of a private branch exchange (PBX). Specifically, the radio telephone system includes a control unit for two-way communication to mobile subscriber units via a radio bus. The radio bus includes elements for transmission and reception of information between the central unit and the mobile subscriber units. Communication from the central unit traverses several radio bus elements until an element is reached that is within radio range of the desired mobile subscriber unit. Communication by radio bus elements utilize time division multiple access (TDMA) wherein time slots may originate from different locations allowing many radio bus elements to transmit to a single receiver, while the mobile subscriber units operate as telephone stations and utilize time division multiplexing (TDM) wherein time slots are transmitted in rigid time sequences originating at a single location.
Frost (U.S. Pat. No. 4,284,848) discloses a switched network telephone subscriber distribution system. Specifically, the system includes a central radio station and radio subscriber stations, each containing two frequency agile duplex radio transceivers. The radio subscriber stations are used as repeaters for calls outside the range of the central radio station. When radio subscriber stations are busy with a call, communication between a particular radio subscriber station and the central radio station is routed on an alternative route utilizing other available radio subscriber stations. In the case of multiple central radio stations, communication from a first central radio station may be transferred to a second central radio station when all subscriber radio stations associated with the first central radio station are being utilized.
The prior art suffers from certain disadvantages. For example, central units tend to require high operating power levels to cover extended ranges, thereby increasing the risk of interference in the communications. Moreover, repeaters are capable of communication only with a single repeater of a different range level, thereby limiting the number of possible paths via which a call may be routed. Further, this restricts communication between repeaters of the same range level.